(1) Field of the Invention
The present invention relates generally to methods for measuring index of refraction, and more particularly to a method for remotely measuring fluctuations in a medium's index of refraction.
(2) Description of the Prior Art
Index of refraction, n, is defined as the ratio between the speed of light in vacuum, c, and the speed of light in a propagation medium, v. When a light source is activated, light propagates from the source at a velocity of 299,792,458 meters per second in a vacuum. Because the speed of light is regarded as a universal constant, the travel time of light is often used as a measurement of distance in terminology such as “light year” and “light second”. Light from a laser propagates in a single direction forming a beam and may be pulsed having a pulse duration as short as 10 femtoseconds. Such a pulsed beam has a physical length or extent in space. This physical length can be estimated as the pulse duration multiplied by the speed of light in vacuum. The length can be calculated exactly by multiplying by the speed of light in the medium; however, the speed of light in the medium may not be exactly known.
Measurement of the index of refraction of a light propagation medium is traditionally accomplished by collecting samples of the medium and using instruments such as an optical interferometer or refractometer to measure the index of refraction. Another approach involves measurement of quantities such as temperature, salinity, and pressure, and then using such quantities in analytical models to calculate the index of refraction. While various methods/instruments (e.g., shadowgraphs and wavefront sensors) have been used to indicate the existence of index of refraction fluctuations, these methods/instruments do not have the ability to remotely sense index of refraction fluctuations.